fix: Resolve CSS bundling in production and lint errors

- Fix golangci-lint errcheck errors by using t.Setenv() instead of os.Setenv()
- Add CSS bundle build step to deploy workflow for production
- Add graceful fallback to modular CSS if bundle doesn't exist
- Remove unused os import from preferences_test.go

.github/workflows/deploy.yml

@@ -46,6 +46,18 @@ jobs:
 
             git pull origin main
 
+            # Build CSS bundle for production
+            echo "🎨 Building CSS bundle..."
+            if command -v lightningcss &> /dev/null; then
+              mkdir -p static/dist
+              lightningcss --bundle --minify static/css/main.css -o static/dist/bundle.min.css
+              echo "✅ CSS bundle created ($(du -h static/dist/bundle.min.css | cut -f1))"
+            else
+              echo "⚠️  lightningcss not found, falling back to modular CSS"
+              # Ensure dist directory doesn't exist so template falls back to main.css
+              rm -rf static/dist
+            fi
+
             # Reapply stashed changes if any (optional - comment out if not needed)
             # if git stash list | grep -q "Auto-stash"; then
             #   echo "♻️  Reapplying stashed changes..."